Absorbent articles surface cross-linked superabsorbent polymer particles made by a method of using ultraviolet radiation and bronsted acids

ABSTRACT

A method of surface cross-linking superabsorbent polymer particles using UV irradiation is provided. The superabsorbent polymer particles for use in the method have a relatively high degree of neutralization. Brønsted acids are selectively applied onto the surface of the superabsorbent polymer particles to selectively facilitate a relatively high number of protonated carboxyl groups at the surface of the superabsorbent polymer particles while the relatively high degree of neutralization in the core of the superabsorbent polymer particles remains substantially unaffected.

FIELD OF THE INVENTION

The present application relates to a method for makingsurface-cross-linked superabsorbent polymer (SAP) particles, usingultraviolet (UV) radiation. The method uses SAP particles with arelatively high degree of neutralization and further applies Brønstedacids. The present application also relates to absorbent articlescomprising SAP particles made by said method.

BACKGROUND OF THE INVENTION

Superabsorbent polymers (SAPs) are well known in the art. They arecommonly applied in absorbent articles, such as diapers, training pants,adult incontinence products and feminine care products to increase theabsorbent capacity of such products while reducing their overall bulk.SAPs are capable of absorbing and retaining amounts of aqueous fluidsequivalent to many times their own weight.

Commercial production of SAPs began in Japan in 1978. The earlysuperabsorbent was a cross-linked starch-g-polyacrylate. Partiallyneutralized polyacrylic acid eventually replaced earlier superabsorbentsin the commercial production of SAPs, and has become the primary polymerin SAPs. SAPs are often applied in form of small particles. Theygenerally consist of a partially neutralized lightly cross-linkedpolymer network, which is hydrophilic and permits swelling of thenetwork once submerged in water or an aqueous solution such asphysiological saline. The cross-links between the polymer chains assurethat the SAP does not dissolve in water.

After absorption of an aqueous solution, swollen SAP particles becomevery soft and deform easily. Upon deformation the void spaces betweenthe SAP particles are blocked, which drastically increases the flowresistance for liquids. This is generally referred to as “gel-blocking”.In gel blocking situations liquid can move through the swollen SAPparticles only by diffusion, which is much slower than flow in theinterstices between the SAP particles.

One commonly applied way to reduce gel blocking is to make the particlesstiffer, which enables the swollen SAP particles to retain theiroriginal shape thus creating or maintaining void spaces between theparticles. A well-known method to increase stiffness is to cross-linkthe carboxyl groups exposed on the surface of the SAP particles. Thismethod is commonly referred to as surface cross-linking.

The art refers, for example, to surface cross-linked and surfactantcoated absorbent resin particles and a method of their preparation. Thesurface cross-linking agent can be a polyhydroxyl compound comprising atleast two hydroxyl groups, which react with the carboxyl groups on thesurface of the SAP particles. In some art, surface cross-linking iscarried out at temperatures of 150° C. or above.

More recently the use of an oxetane compound and/or an imidazolidinonecompound for use as surface cross-linking agent has been disclosed. Thesurface cross-linking reaction can be carried out under heat, whereinthe temperature is preferably in the range of 60° C. to 250° C.Alternatively, the surface cross-linking reaction can also be achievedby a photo-irradiation treatment, preferably using ultraviolet rays.

A drawback of the commercial surface cross-linking process describedabove is that it takes relatively long, commonly at least about 30 min.However, the more time is required for the surface cross-linkingprocess, the more surface cross-linking agent will penetrate into theSAP particles, resulting in increased cross-linking inside theparticles, which has a negative impact on the capacity of the SAPparticles. Therefore, it is desirable to have short process times forsurface cross-linking. Furthermore, short process times are alsodesirable with respect to an overall economic SAP particle manufacturingprocess.

Another drawback of common surface cross-linking processes is, that theytake place only under relatively high temperatures, often around 150° C.or above. At these temperatures, not only the surface cross-linkerreacts with the carboxyl groups of the polymer, but also other reactionsare activated, such as anhydride-formation of neighbored carboxyl groupswithin or between the polymer chains, and dimer cleavage of acrylic aciddimers incorporated in the SAP particles. Those side reactions alsoaffect the core, decreasing the capacity of the SAP particles. Inaddition, exposure to elevated temperatures can lead to colordegradation of the SAP particles. Therefore, these side reactions aregenerally undesirable.

The use of acids for the production of water-absorbent agents isdisclosed in U.S. Pat. No. 5,610,208. The patent refers to a method forproducing water-absorbent agents which comprises mixing awater-absorbent resin containing a carboxyl group with an additive of atleast one member selected from the group consisting of inorganic acids,organic acids, and polyamino acids and a cross-linking agent capable ofreacting with the carboxyl group. The mixture is subjected to a heattreatment at a temperature in the range of from 100° C. to 230° C.

SAPs known in the art are typically partially neutralized, for example,with sodium hydroxide. However, neutralization has to be carefullybalanced with the need for surface cross-linking: The surfacecross-linking agents known in the art react with free carboxyl groupscomprised by the polymer chains at relatively high speed but react witha neutralized carboxyl groups only very slowly. Thus, given carboxylgroups can either be applied for surface cross-linking or forneutralization, but not for both. Surface cross-linking agents known inthe art preferably react with the chemical group carboxyl groups, theydo not react with aliphatic groups.

In the process of making SAP particles, neutralization of free carboxylgroups typically comes first, before surface cross-linking takes place.Indeed, the neutralization step is often carried out in the verybeginning of the process, before the monomers are polymerized andcross-linked to form the SAP. Such a process is named“pre-neutralization process”. Alternatively, the SAP can be neutralizedduring polymerization or after polymerization (“post-neutralization”).Furthermore, a combination of these alternatives is also possible.

The overall number of free carboxyl groups on the outer surface of theSAP particles is not only limited by the foregoing neutralization butthe free carboxyl groups are also believed to be not homogeneouslydistributed. Hence, it is currently difficult to obtain SAP particleswith evenly distributed surface cross-linking. On the contrary, oftenSAP particles have regions of rather dense surface cross-linking, forexample, with a relatively high number of surface cross-links, andregions of sparsely surface cross-linking. This inhomogeneity has anegative impact on the desired overall stiffness of the SAP particles.

In one embodiment, a method of making SAP particles with evenlydistributed, homogenous surface cross-linking is provided while usingSAP particles having a high degree of neutralization.

In another embodiment, an economic method of surface cross-linking SAPparticles is provided.

Moreover, it is difficult to obtain SAP particles having both,sufficient stiffness to avoid gel blocking (sometimes referred to as“gel strength”) and sufficient swelling capacity (sometimes referred toas “gel volume”). Typically, increasing the gel strength of the SAPparticles has a negative impact on the gel volume and vice versa.

In another embodiment, the surface cross-links are restricted to thevery surface of the SAP particles in order to minimize the decrease incapacity. Thus, the core of the SAP particles should not be considerablyaffected and the additional cross-links introduced in the core should bekept to a minimum.

In another embodiment, a method of surface cross-linking SAP particlesis provided, which can be carried out quickly to increase the efficiencyof the method.

In another embodiment, a method of surface cross-linking SAP particlesis provided, which can be carried out at moderate temperatures in orderto reduce undesired side reactions, such as anhydride-formation anddimer cleavage.

SUMMARY OF THE INVENTION

In one embodiment, a method of surface cross-linking superabsorbentpolymer particles is provided which comprises the steps of:

-   -   a) providing superabsorbent polymer particles having a surface        and a core and having a degree of neutralization of more than 60        mol-%;    -   b) applying one or more Brønsted acids onto the surface of the        superabsorbent polymer particles; and either        -   c1) exposing the superabsorbent polymer particles to            irradiation with vacuum UV radiation having a wavelength            from about 100 nm to about 200 nm; or        -   c2) exposing the superabsorbent polymer particles to            irradiation with UV radiation having a wavelength from about            201 nm to about 400 nm and wherein further to the Brønsted            acids, radical former molecules are applied to the surface            of the superabsorbent polymer particles.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, the SAPs comprise a homo-polymer of partiallyneutralized α,β-unsaturated carboxylic acid or a copolymer of partiallyneutralized α,β-unsaturated carboxylic acid copolymerized with a monomerco-polymerizable therewith. Furthermore, the homo-polymer or copolymercomprised by the SAP comprises aliphatic groups, wherein at least someof the aliphatic groups are at least partially comprised by the surfaceof the SAP particles.

SAPs are available in a variety of chemical forms, including substitutedand unsubstituted natural and synthetic polymers, such as carboxymethylstarch, carboxymethyl cellulose, and hydroxypropyl cellulose; nonionictypes such as polyvinyl alcohol, and polyvinyl ethers; cationic typessuch as polyvinyl pyridine, polyvinyl morpholinione, andN,N-dimethylaminoethyl or N,N-diethylaminopropyl acrylates andmethacrylates, and the respective quaternary salts thereof. Typically,SAPs useful herein have a multiplicity of anionic, functional groups,such as sulfonic acid, and more typically carboxyl groups. Examples ofpolymers suitable for use herein include those, which are prepared frompolymerizable, unsaturated, acid-containing monomers. Thus, suchmonomers include the olefinically unsaturated acids and anhydrides thatcontain at least one carbon-to-carbon olefinic double bond. Morespecifically, these monomers can be selected from olefinicallyunsaturated carboxylic acids and acid anhydrides, olefinicallyunsaturated sulfonic acids, and mixtures thereof.

Some non-acid monomers can also be included, usually in minor amounts,in preparing SAPs. Such non-acid monomers can include, for example, thewater-soluble or water-dispersible esters of the acid-containingmonomers, as well as monomers that contain no carboxylic or sulfonicacid groups at all. Optional non-acid monomers can thus include monomerscontaining the following types of functional groups: carboxylic acid orsulfonic acid esters, hydroxyl groups, amide-groups, amino groups,nitrile groups, quaternary ammonium salt groups, aryl groups (e.g.,phenyl groups, such as those derived from styrene monomer). Thesenon-acid monomers are well-known materials and are described in greaterdetail, for example, in U.S. Pat. No. 4,076,663 and in U.S. Pat. No.4,062,817.

Olefinically unsaturated carboxylic acid and carboxylic acid anhydridemonomers include the acrylic acids typified by acrylic acid itself,methacrylic acid, ethacrylic acid, α-chloroacrylic acid, α-cyanoacrylicacid, β-methylacrylic acid (crotonic acid), α-phenylacrylic acid,β-acryloxypropionic acid, sorbic acid, α-chlorosorbic acid, angelicacid, cinnamic acid, p-chlorocinnamic acid, β-sterylacrylic acid,itaconic acid, citroconic acid, mesaconic acid, glutaconic acid,aconitic acid, maleic acid, fumaric acid, tricarboxyethylene and maleicacid anhydride.

Olefinically unsaturated sulfonic acid monomers include aliphatic oraromatic vinyl sulfonic acids such as vinylsulfonic acid, allyl sulfonicacid, vinyl toluene sulfonic acid and styrene sulfonic acid; acrylic andmethacrylic sulfonic acid such as sulfoethyl acrylate, sulfoethylmethacrylate, sulfopropyl acrylate, sulfopropyl methacrylate,2-hydroxy-3-methacryloxypropyl sulfonic acid and2-acrylamide-2-methylpropane sulfonic acid.

In one embodiment, SAPs contain carboxyl groups. These polymers comprisehydrolyzed starch-acrylonitrile graft copolymers, partially neutralizedhydrolyzed starch-acrylonitrile graft copolymers, starch-acrylic acidgraft copolymers, partially neutralized starch-acrylic acid graftcopolymers, saponified vinyl acetate-acrylic ester copolymers,hydrolyzed acrylonitrile or acrylamide copolymers, slightly networkcross-linked polymers of any of the foregoing copolymers, partiallyneutralized polyacrylic acid, and slightly network cross-linked polymersof partially neutralized polyacrylic acid, partially neutralizedpolymethacrylic acid, and slightly network cross-linked polymers ofpartially neutralized polymethacrylic acid. These polymers can be usedeither solely or in the form of a mixture of two or more differentpolymers, that when used as mixtures, individually do not have to bepartially neutralized, whereas the resulting copolymer has to be.Examples of these polymer materials are disclosed in U.S. Pat. No.3,661,875, U.S. Pat. No. 4,076,663, U.S. Pat. No. 4,093,776, U.S. Pat.No. 4,666,983, and U.S. Pat. No. 4,734,478.

In one example, polymer materials for use herein are slightly networkcross-linked polymers of partially neutralized polyacrylic acids,slightly network cross-linked polymers of partially neutralizedpolymethacrylic acids, their copolymers and starch derivatives thereof.SAPs comprise partially neutralized, slightly network cross-linked,polyacrylic acid (for example, poly (sodium acrylate/acrylic acid)). TheSAPs for use in one embodiment are at least about 60% to about 95%, inanother embodiment at least about 65% to about 95%, in anotherembodiment at least about 70% to about 95%, and in yet anotherembodiment from about 75% to about 95% neutralized. Networkcross-linking renders the polymer substantially water-insoluble and, inpart, determines the absorptive capacity of the hydrogel-formingabsorbent polymers. Processes for network cross-linking these polymersand typical network cross-linking agents are described in greater detailin U.S. Pat. No. 4,076,663.

A suitable method for polymerizing α,β-unsaturated carboxylic acidmonomers is aqueous solution polymerization, which is well known in theart. An aqueous solution comprising α,β-unsaturated carboxylic acidmonomers and polymerization initiator is subjected to a polymerizationreaction. The aqueous solution may also comprise further monomers, whichare co-polymerizable with α,β-unsaturated carboxylic acid monomers. Atleast the α,β-unsaturated carboxylic acid has to be partiallyneutralized, either prior to polymerization of the monomers, duringpolymerization or post polymerization.

The monomers in aqueous solution are polymerized by standard freeradical techniques, commonly by using a photoinitiator for activation,such as ultraviolet (UV) light activation. Alternatively, a redoxinitiator may be used. In this case, however, increased temperatures aredesirable.

In one example, the water-absorbent resin is lightly cross-linked torender it water-insoluble. The desired cross-linked structure may beobtained by the co-polymerization of the selected water-soluble monomerand a cross-linking agent possessing at least two polymerizable doublebonds in the molecular unit. The cross-linking agent is present in anamount effective to cross-link the water-soluble polymer. The amount ofcross-linking agent is determined by the desired degree of absorptioncapacity and the desired strength to retain the absorbed fluid, that is,the desired absorption under load. Typically, the cross-linking agent isused in amounts ranging from about 0.0005 to about 5 parts by weight per100 parts by weight of monomers (including α,β-unsaturated carboxylicacid monomers and possible co-monomers) used. If an amount over 5 partsby weight of cross-linking agent per 100 parts is used, the resultingpolymer has a too high cross-linking density and exhibits reducedabsorption capacity and increased strength to retain the absorbed fluid.If the cross-linking agent is used in an amount less than 0.0005 partsby weight per 100 parts, the polymer has a too low cross-linking densityand when contacted with the fluid to be absorbed becomes rather sticky,water-soluble and exhibits a low absorption performance, particularlyunder load. The cross-linking agent will typically be soluble in theaqueous solution.

Alternatively to co-polymerizing the cross-linking agent with themonomers, it is also possible to cross-link the polymer chains in aseparate process step after polymerization.

After polymerization, cross-linking and partial neutralization, the wetSAPs are dehydrated (i.e., dried) to obtain dry SAPs. The dehydrationstep can be performed by heating the viscous SAPs to a temperature ofabout 120° C. for about 1 or 2 hours in a forced-air oven or by heatingthe viscous SAPs overnight at a temperature of about 60° C. The contentof residual water in the SAP after drying predominantly depends ondrying time and temperature. In one embodiment, “dry SAP” refers to SAPwith a residual water content of from about 0.5% by weight of dry SAP upto about 50% by weight of dry SAP, in another embodiment from about 0.5%to about 45% by weight of dry SAP, in another embodiment from about 0.5%to about 30%, in another embodiment from about 0.5% to about 15%, and inyet another embodiment from about 0.5% to about 5%. If not explicitlysaid to be otherwise, in the following the term “SAP particles” refersto dry SAP particles.

The SAPs can be transferred into particles of numerous shapes. The term“particles” refers to granules, fibers, flakes, spheres, powders,platelets and other shapes and forms known to persons skilled in the artof SAPs. For example, the particles can be in the form of granules orbeads, having a particle size of from about 10 μm to about 1000 μm, inanother embodiment from about 100 μm to about 1000 μm. In anotherembodiment, the SAPs can be in the shape of fibers, for example,elongated, acicular SAP particles. In those embodiments, the SAP fibershave a minor dimension (i.e., diameter of the fiber) of less than about1 mm, in another example less than about 500 μm, and in yet anotherexample less than about 250 μm down to about 50 μm. In one embodiment,the length of the fibers is from about 3 mm to about 100 mm. The fiberscan also be in the form of a long filament that can be woven.

The SAP particles of one embodiment have a core and a surface. In oneembodiment, the dry SAP particles undergo a surface cross-linkingprocess step, for example, they are cross-linked in their surface whilethe number of cross-links in the core of the particle is notsubstantially increased by the method of the invention.

The term “surface” describes the outer-facing boundaries of theparticle. For porous SAP particles, exposed internal surfaces may alsobelong to the surface. As used herein, the term “surface” of the SAPparticles refers to the complete and continuous outwardly facing 6%volume of the dry SAP particle, whereas “core” refers to 94% of thevolume and comprises the inner regions of the dry SAP particle.

Surface cross-linked SAP particles are well known in the art. In surfacecross-linking methods of the prior art, a surface cross-linker isapplied to the surface of the SAP particles. In a surface cross-linkedSAP particle the level of cross-links in the surface of the SAP particleis considerably higher than the level of cross-links in the core of theSAP particle.

Commonly applied surface cross-linkers are thermally activatable surfacecross-linkers. As used herein, the term “thermally activatable surfacecross-linkers” refers to surface cross-linkers, which only react uponexposure to increased temperatures, typically around 150° C. Thermallyactivatable surface cross-linkers known in the prior art are, forexample, di- or polyfunctional agents that are capable of buildingadditional cross-links between the polymer chains of the SAPs. Typicalthermally activatable surface cross-linkers include, for example, di- orpolyhydric alcohols, or derivatives thereof, capable of forming di- orpolyhydric alcohols. Examples of such agents are alkylene carbonates,ketales, and di- or polyglycidlyethers. Moreover, (poly)glycidyl ethers,haloepoxy compounds, polyaldehydes, polyoles and polyamines are alsowell known thermally activatable surface cross-linkers. Thecross-linking is, for example, formed by an esterification reactionbetween a carboxyl group (comprised by the polymer) and a hydroxyl group(comprised by the surface cross-linker). Typically, a relatively bigpart of the carboxyl groups of the polymer chain is neutralized prior tothe polymerization step, commonly only few carboxyl groups are availablefor this surface cross-linking process known in the art. For example, ina 70% percent neutralized polymer only 3 out of 10 carboxylic groups areavailable for covalent surface cross-linking.

In one embodiment, the method is used for surface cross-linking of SAPparticles. Hence, the polymer chains comprised by the SAP particlesalready have been cross-linked by a cross-linker known in the art,comprising at least two polymerizable double bonds in the molecule unit.

In another embodiment, direct covalent bonds between carbon atomscomprised in the backbone of different polymer chains are formed in thesurface of the SAP particles.

Optionally, surface cross-linking molecules may also be used. In suchembodiments wherein surface cross-linking molecules are added to the SAPparticles, additional covalent bonds are formed between the polymerchains comprised in the surface of the SAP particles. These additionalcovalent bonds comprise the reaction product of said surfacecross-linking molecules.

As used herein, the term “direct covalent bond” is a covalent bondwherein polymer chains are bound to each other only via a covalent bondwith no intermediate atoms, such as atoms comprised by a cross-linkingmolecule. In contrast, known cross-linking reactions between polymerchains always result in covalent bonds between these polymer chains,wherein the reaction product of the cross-linking molecule is built inbetween the polymer chains. Thus, known surface cross-linking reactionsdo not result in a direct covalent bond but in an indirect covalent bondcomprising the reaction product of the cross-linking molecule. Thedirect covalent bond is formed between a carbon atom in the backbone ofa first polymer chain and a carbon atom in the backbone of a secondpolymer chain. The bonds are formed intra-particulate within the SAPparticle, more specifically, they are formed in the surface of the SAPparticles, while the core of the SAP particles is substantially free ofsuch direct covalent bonds.

The “backbone” of a polymer chain refers to those carbon atoms whichimmediately form the polymer chain. Principally, if a reaction resultedin the removal of a carbon atom, which is part of the polymer chainbackbone, this reaction would also result in the break of the polymerchain on the position, where this carbon atom had previously been builtinto the polymer chain.

Optionally, surface cross-linking molecules may also be used. In suchembodiments wherein surface cross-linking molecules are added to the SAPparticles, additional covalent bonds are formed between the polymerchains comprised in the surface of the SAP particles. These additionalcovalent bonds comprise the reaction product of said surfacecross-linking molecules.

The cross-linking of different polymer chains is not intended to bonddifferent SAP particles to each other. Thus, the method according to oneembodiment does not lead to any appreciable inter-particulate bondsbetween different SAP particles but only results in intra-particulatedirect covalent bonds within an SAP particle. If present, suchinter-particulate direct covalent bonds would hence require additionalinter-particulate cross-linking materials.

In another embodiment, the method which directly bonds polymer chains toeach other by a covalent bond between two carbon atoms can be appliedfor surface cross-linking SAP particles instead of or additional toconventional surface cross-linking.

Radiation Activatable Radical Former Molecules

In one embodiment, if UV radiation having a wavelength from 100 nm to200 nm (vacuum UV, hereinafter referred to as VUV) is used, radiationactivatable radical former molecules may optionally be applied toincrease the efficiency of the surface cross-linking. However, the useof such radical formers is not mandatory for VUV and may, indeed, beomitted to reduce costs, as the radical formers may substantially add tothe total costs of the surface cross-linking method. Due to the use ofVUV, the radical formers are not necessarily required to initiate thesurface cross-linking reaction.

In another embodiment, if UV radiation having a wavelength from 200 nmto 400 nm is used, radical former molecules have to be applied as thesurface cross-linking reaction, which is a radical reaction, cannot beinitiated otherwise.

The radiation activatable radical former molecules (hereinafter calledradical formers) are able to form carbon centered radicals located inthe polymer backbone of polymer chains comprised in the surface of theSAP particles. This reaction takes place upon UV irradiation. Two ofthese carbon centered radicals comprised in different polymer chains areable to react with each other and thereby form a direct covalent bondbetween the polymer chains.

Upon irradiation, some of the radical formers form, in a first step, anintermediate radical, which is typically oxygen-centered, and which may,in a second step, react with a carbon atom comprised in the polymerbackbone in the surface of the SAP particle to form a carbon centeredradical in the polymer backbone.

In principle, any photo-initiator which is typically used to start thepolymerization of vinyl monomers can be applied as a radical former forsurface cross-linking according to various embodiments. Suchphotoinitiators typically serve to trigger radical chain polymerizationsof vinyl monomers. It is believed that the reactive intermediatespecies, which is formed upon irradiation of the photoinitiator with UVradiation, is capable of abstracting hydrogen atoms from C—H bonds of Catoms comprised by the polymer backbone of polymer chains in the surfaceof the SAP particle (therewith initiating the cross-linking according toone embodiment).

In another embodiment, the radiation activatable radical former moleculecomprises a peroxo bridge (O—O), which is homolytically cleaved upon UVirradiation (so-called photo-fragmentation).

However, reactive intermediate species can also be ketones which—upon UVirradiation—have been transferred into short-lived, a so-called excitedtriplet state. The keton in the triplet-state is also capable ofabstracting hydrogen from C—H bonds of C atoms comprised by the polymerbackbone whereby the ketone is converted into an alcohol (so-calledphoto reduction).

In one embodiment, the radical former is water soluble. The watersoluble radical former should exhibit a solubility in water of at leastabout 1 wt %, in another embodiment at least about 5 wt %, and in yetanother embodiment at least about 10 wt % at 25° C.

Radical formers, which are not initially water soluble, can be renderedwater soluble by derivatization, for example, by introducing a chargedgroup into the molecular structure, such as carboxylate or ammonium. Asan example, benzophenone can be easily derivatized into benzoyl benzoicacid. However, in one example the radical formers are inherently watersoluble, i.e., the introduction of a functional group is not required.Typical, inherently water soluble radiation activatable radical formersare peroxides like alkali-metal or other inorganic peroxodisulfates, orderivatized organic peroxodisulfates. Water-soluble azo-initiators canbe used as well (such as the commercially available V-50 or VA-086, WakoSpecialty Chemicals). Inorganic peroxides typically fulfill therequirement of water solubility, while organic compounds typicallyrequire derivatization. In one example, the water-soluble radical formeris sodium peroxodisulfate.

The advantage of providing the radical former in an aqueous solution(and hence, the advantage of using a water-soluble radical former) istwo-fold: On the one hand, the aqueous solution facilitates an efficientwetting of the SAP particle surface. Thus, the radical former moleculesare actually transported into the particle surface, where they initiatethe surface cross-linking reaction.

On the other hand, efficient wetting of the SAP particle surfaceenhances the chain mobility of the polymer chains comprised in thesurface of the SAP particles. This facilitates the bimolecular reactionbetween the carbon atoms comprised in the polymer backbone and thereactive intermediate species, into which the radical former istransformed upon irradiation. This effect is particularly advantageousfor SAP particles comprised of poly(meth)acrylic acid, which are in factthe most widely used SAP particles of today. Polyacrylic acid possessesa glass transition temperature of 106° C. and the sodium salt ofpolyacrylic acid, at a neutralization degree of 100%, has a glasstransition temperature of above 200° C. while, in one embodiment thesurface cross-linking is typically carried out at temperatures below100° C. In the presence of water, the glass transition temperature ofpartly neutralized polyacrylic acid can be significantly decreased. Forexample, the glass transition temperature of a 65% neutralized sodiumpolyacrylate can be reduced from ca. 150° C. in the presence of 5 wt %water to below room temperature in the presence of 35 wt % water.However, to make use of this effect, the actual local waterconcentration directly in the surface of the SAP particle is important.

In one embodiment, to ensure that the cross-linking is actuallyrestricted to the surface of the SAP particles, the water should beprevented from evenly distributing throughout the whole particle volumevia diffusion. Therefore, the UV irradiation step should follow notlater than one hour after the aqueous solution comprising the radicalformer has been applied onto the SAP particles, in another embodimentnot later than 10 minutes, and in yet another embodiment not later than1 minute.

In one example, water-soluble radical formers are utilized, as organicsolvents are typically more expensive than water and are also moreproblematic from an environmental standpoint. However, organic radialformers which have not been rendered water-soluble via theabove-described derivitization may also be used and can be applied in anorganic solvent rather than in water. Examples are benzophenone or anyother suitable ketone which is known to undergo photoreduction whenirradiated with UV radiation. A further example is dibenzoyl peroxide orany other organic peroxide which is known to undergo photo fragmentationwhen irradiated with UV radiation.

In one embodiment, the radical former is applied in amounts of less thanabout 25% by weight of SAP particles, in another embodiment in amountsof less than about 15%, and in yet another embodiment in amounts fromabout 1% to about 5%. The radical former is typically applied in aqueoussolution. In another embodiment, the radical former and the water can beadded in two steps, but both have to be present on the surface duringirradiation. In one embodiment, the amount of water is less than about25% by weight of SAP particles, in another embodiment less than about15%, and in yet another embodiment from about 5% to about 10%. Foreconomic reasons, the amount of water added may be kept as low aspossible to shorten or entirely avoid a drying step after the surfacecross-linking.

Surface Cross-Linking Molecules

The surface cross-linking molecule is any compound having at least twofunctional groups which can react with the aforementionedcarbon-centered radicals located in the backbone of the polymer chainscomprised in the surface of the SAP particles. Upon reaction of thefunctional group in the surface cross-linking molecule with thecarbon-centered radical, a new covalent bond is formed, grafting thecross-linking molecule onto the polymer backbone.

In one embodiment, the functional groups of the surface cross-linkingmolecules are C═C double bonds. In another embodiment, a cross-linkingmolecule comprises more than two C═C double bonds. Alternatively, thefunctional groups can also be CH—X moieties, with X being a hetero atom.An example of a CH—X moiety is an ether, CH—O—R, with R being an alkylresidue.

In one embodiment, cross-linking molecules are polyfunctional allyl andacryl compounds, such as triallyl cyanurate, triallyl isocyanurate,trimethylpropane tricrylate or other triacrylate esters, pentaerythritoltriallyl ether, pentaerythritol tetraallyl ether, butanediol diacrylate,pentaerythritol tetraacrylate, tetra allylorthosilicate,di-pentaerythritol pentaacyralate, di-pentaerythritol hexaacyralate,ethyleneglycol diacrylate, ethyleneglycol dimethacrylate, tetra allyloxyethane, diallyl phthalate, diethyleneglycol diacrylate,allylmethacrylate, triallylamine, 1,1,1-trimethylolpropane triacrylate,triallyl citrate, or triallyl amine.

In another embodiment, the cross-linking molecules are selected from thegroup consisting of squalene, N,N′ methylenebisacrylamide,icosa-pentaenic acid, sorbic acid or vinyl terminated silicones.

In one embodiment, compounds with allylic double bonds are utilizedrather than compounds with acrylic double bonds. In another embodiment,the cross-linking molecule is diallyl dimethyl ammonium chloride.

If surface cross-linking molecules are applied, they should be added,for example, by spray application in a solution with an inert solvent(that can be optionally evaporated) before the SAP particles undergo UVirradiation. The surface cross-linking molecules can be applied in anorganic solvent like dichloromethane which is evaporated directly afterapplication. In embodiments, wherein the SAP particles are moisturized,the surface cross-linking molecules can also be applied together withthe water as a suspension or, if the surface cross-linking molecules arewater soluble, as a solution.

Moreover, in embodiments wherein surface cross-linking molecules areapplied together with radical formers, the molar ratio of surfacecross-linking molecules to radical former is in the range of from about0.2 to about 5, in another embodiment from about 0.33 to about 3, and inyet another embodiment from about 1 to about 3.

In embodiments, wherein only surface cross-linking molecules are usedwithout additional use of radical formers (only applicable for VLUVirradiation), the surface cross-linking molecules are preferably appliedin a concentration from about 0.1% to about 10% by weight of dry SAPparticles and in another embodiment from about 1% to about 5%.

In one embodiment, the surface cross-linking compound is water-soluble,so that it can be applied with the aqueous solution comprising theradical former (optional for use of VUV). If a water-insoluble surfacecross-linking molecules is applied, it may be emulsified or suspended inthe aqueous solution comprising the radical former or be appliedseparately. Water-insoluble surface cross-linking molecules can also beapplied in an organic solvent like dichloromethane which is evaporateddirectly after application.

The surface cross-linking molecules and/or the radical former may besprayed onto the SAP particles by means of a fluidized-bed sprayingchamber. Simultaneously IR-irradiation may be applied to accomplishdrying. Instead or in combination with IR-light, any conventional dryingequipment can be used for drying. However, in certain embodiments littleor no drying is required, for example, in cases, where only smallamounts of surface cross-linking molecules and/or the radical former areapplied, dissolved in small amounts of solution.

In one embodiment, the surface cross-linking molecules and/or theradical formers are always applied onto the SAP particles prior to UVirradiation or simultaneously with UV irradiation.

Reaction Mechanism without Radical Formers and without SurfaceCross-Linking Molecules (Applicable for Use of VUV Only)

Several mechanisms can be distinguished that contribute to the formationof the intermediate carbon-centred radicals. To some degree, thosemechanisms may take place simultaneously.

Upon irradiation with UV having a wavelength from about 100 nm to about200 nm (vacuum UV, in the following called VUV), hydroxyl radicals aregenerated from water molecules via homolytic cleavage of O—H bonds.Those highly reactive, short-lived species are capable of abstractinghydrogen atoms from the carbon-hydrogen bonds (C—H bonds) comprised inthe backbone of the polymer chains in the surface of the SAP particles,resulting in the formation of said carbon-centred radicals.

It is also possible that instead of abstracting a hydrogen atom from acarbon-hydrogen bond comprised in the backbone of the polymer chain, acomplete carboxyl group is abstracted from the polymer chain(decarboxylation). As a result of this reaction, a carbon-centredradical is formed in the backbone of a polymer chain comprised in thesurface of the SAP particle.

The water molecules can, for example, be the residual water moleculescomprised within the dry SAP particles but can also be provided byslightly moisturizing the SAP particles via a spray application or,preferably, as water vapor. Moisturizing may, for example, be advisableif SAP particles with relatively low residual water contents (below 0.5%by weight of the dry SAP particles) are used.

Reaction Mechanism with Radical Formers (Optional for Use of VUV) andwith Optional Surface Cross-Linking Molecules:

The radical former molecules undergoing photo-fragmentation comprise alabile bond. Upon UV irradiation, the labile bond breaks, whereby tworadicals (R_(a). and R_(b).) are formed.

This homolytic cleavage may result in two identical radicals, if thelabile bond comprised by the radical former molecule (so-calledprecursor molecule) divides the molecule into two identical parts.Alternatively, the homolytic cleavage may result in two differentradicals.

The radicals, which have been formed, can now react with an aliphaticC—H group comprised in the backbone of the polymer chains in the surfaceof the SAP particle forming a carbon-centered radical in the polymerbackbone. Two such carbon-centered radicals can react with each other toform a direct covalent bond between the carbon atoms comprised in thepolymer backbone.

Again, it is also possible that instead of abstracting a hydrogen atomfrom a carbon-hydrogen bond comprised in the backbone of the polymerchain, a complete carboxyl group is abstracted from the polymer chain(decarboxylation). As a result of this reaction, a carbon-centredradical is formed in the backbone of a polymer chain comprised in thesurface of the SAP particle.

Optionally, surface cross-linking molecules may be additionally used. Insuch embodiments, the radicals formed from the radical former molecule,can react with one of the C═C double bonds comprised by thecross-linking molecule to form a radical consisting of the reactionproduct of the cross-linking molecule and the initial radical.

The carbon-centered radical within the polymer chain segment can reactwith this radical. The reaction product of this reaction is a polymerchain wherein the reaction products of the radical former molecule andthe cross-linking molecule are covalently bound to a carbon atom of thepolymer backbone.

Thereafter, the radicals formed from the radical former molecule, canreact with the second of the C═C double bonds of the cross-linkingmolecule.

To form the cross-link between two polymer chains, the carbon-centeredradical combines with another carbon centered radical comprised inanother polymer chain in the surface of the same SAP particle.

Hence, contrary to the reaction described above for VUV only, wherein noradical formers or surface cross-linking molecules are applied, thereaction involving the additional use of radical formers and surfacecross-linking molecules does not result in a direct covalent bondbetween two carbon atoms comprised in the backbone of two differentpolymer chains within the surface of a SAP particle. However, if radicalformers and surface cross-linking molecules are additionally used, thereactions described above leading to a direct covalent bond willadditionally take place.

Moreover, it is possible to use only radical formers, in which casecarbon centered radicals in the polymer backbone are formed. In suchembodiments, only direct covalent bonds are formed and the radicalformer is not covalently bonded to the surface of the SAP particles.

In embodiments using VUV, it is also possible to apply only surfacecross-linking molecules without additionally using radical formers. Inthese embodiments, the carbon-centered radical formed in the polymerbackbone comprised in the surface of a SAP particle upon VUVirradiation, reacts with one of the C═C double bonds of the surfacecross-linking molecule. Thereby, the surface cross-linking molecule iscovalently bound to the surface of the SAP particles and a radical isinduced at one of the two C atoms, which have been comprised by theformer C═C double bond of the surface cross-linking molecule. Thisradical is capable of abstracting again a hydrogen atom from another(neighboring) polymer chain within the surface of the SAP particle, thusresulting in another carbon-centered radical formed in the polymerbackbone of this other polymer chain. This carbon centered radical cannow react with the second C═C double bond comprised in the surfacecross-linking molecule, which is already covalently bound to the SAPparticle via the radical reaction, that has comprised the first C═Cdouble bond. As a result, two polymer chains of the SAP particle arecross-linked via the reaction product of the surface cross-linkingmolecule.

The net reaction when using radical former molecules undergoingphoto-fragmentation upon irradiation is the formation of a cross-linkbetween two polymer chain segments, wherein the cross-link comprises thereaction product of one cross-linking molecule with two C═C double bondsand two radical former molecules.

With the additional use of surface cross-linking molecules theefficiency of the reaction can be further enhanced due to shorterreaction times: Without wanting to be bound by theory, it is believedthat the rate determining step of a UV irradiation initiated surfacecross-linking reaction in the absence of surface cross-linking moleculesis the recombination of two carbon-centered radicals, forming a directcovalent bond between two carbon atoms comprised in two differentpolymer chains. This recombination follows a kinetic law of a secondorder, i.e., the reaction rate is proportional to the concentrations ofboth reactants (i.e., the two combining carbon-centered radicals)multiplied with each other.

If, however, surface cross-linking molecules are added, it is believed,that the reaction between the radical formed from the surfacecross-linking molecule and the carbon-centered radical comprised in thepolymer chain follows a kinetic law of pseudo-first order, for example,the reaction rate is only proportional to the concentration of thecarbon-centered radical, since the concentration of the second reactionpartner, for example, the radicals formed from the surface cross-linkingmolecule, is so high that it can be regarded as constant throughout thereaction. Reactions of pseudo-first order kinetics are known to bekinetically favored versus reactions of second order kinetics, forexample, they have a higher reaction speed.

Alternatively to radical former molecules undergoing photo-fragmentationit is also possible to use radical former molecules undergoingphoto-reduction upon irradiation comprise carbonyl groups. In oneembodiment, such radical former molecules are ketones.

Upon UV irradiation, the radical former molecules of this type aretransferred in an “excited state” (triplet state). Hence, they are notyet transformed into a radical, but are much more reactive than prior toirradiation.

In the next step, the radical former molecule in its excited statereacts with an aliphatic C—H group comprised in the backbone of apolymer chain in the surface of the SAP particle and abstracts ahydrogen radical, thereby forming a carbon-centered radical at thispolymer chain and a ketyl radical.

The ketyl radical can now react with one of the C═C double bonds of thecross-linking molecule.

Alternatively (or exclusively in embodiments which do not use surfacecross-linking molecules) two ketyl radicals can recombine with oneanother to form a so-called pinacol, for example, benzpinacol, forbenzophenone as initiator.

In one embodiment comprising both types of radical former molecules, thesurface cross-linking molecules comprise more than two C═C double bonds.In these embodiments, more than two polymer chain segments can becross-linked to each other, following the reaction mechanism describedabove. In these embodiments, the number of reaction products of radicalformer molecules comprised by the cross-link equals the number of C═Cdouble bonds comprised by the cross-linking molecule.

According to one embodiment, only one type of cross-linking moleculesmay be used or, alternatively, two or more chemically differentcross-linking molecules can be applied. Likewise, the only one type ofradiation activatable radical former molecule can be used or,alternatively, two or more chemically different radiation activatableradical former molecules can be applied.

To ensure that SAP particles with evenly distributed surfacecross-linking are obtained, the radical former (optionally applied ifVUV is used, mandatory for all other UV radiation) and the optionalsurface cross-linking molecules have to be distributed evenly on the SAPparticle. Therefore, the surface cross-linker is preferably applied byspraying onto the SAP particles.

Compared to the surface cross-linking known from the prior art, thesurface cross-linking according to one embodiment is significantlyfaster. Prior art surface cross-linking reactions carried out underincreased temperatures commonly take up to 45 minutes. This timeconsuming process step renders the manufacturing process of SAPparticles less economic than desirable. In contrast, the cross-linkingprocess according to one embodiment can be carried out within asignificantly shorter reaction time, typically within minutes, andhence, enables an overall improvement with respect to manufacturingtimes of the SAP particles. This results in lower energy costs andhigher throughput.

Furthermore, as the surface cross-linking reaction proceeds quickly, theradical former molecules (optionally applied if VUV is used)and—optionally—surface cross-linking molecules applied on the surface ofthe SAP particles have less time to penetrate inside the SAP particles.Hence, compared to prior art surface cross-linking, it is easier toactually restrict surface cross-linking to the surface of the SAPparticles and to avoid undesired further cross-linking reactions in thecore of the SAP particles.

In one embodiment, the α,β-unsaturated carboxylic acid monomers areoften neutralized prior to the polymerization step (pre-neutralization).This step is referred to as the neutralization step. Compounds, whichare useful to neutralize the acid groups of the monomers, are typicallythose, which will sufficiently neutralize the acid groups without havinga detrimental effect on the polymerization process. Such compoundsinclude alkali metal hydroxides, alkali metal carbonates andbicarbonates. Preferably, the material used for neutralization of themonomers is sodium- or potassium-hydroxide, or sodium- orpotassium-carbonate. As a result, the carboxyl groups comprised by theα,β-unsaturated carboxylic acid of the polymer are at least partiallyneutralized. In case sodium hydroxide is used, neutralization results insodium acrylate, which dissociates in water into negatively chargedacrylate monomers and positively charged sodium ions. As the surfacecross-linkers known in the art react with the carboxyl groups of thepolymer, the degree of neutralization has to be balanced with the needto surface cross-link, because both process steps make use of thecarboxyl groups.

If the final SAP particles are in the swollen state, after they absorbedaqueous solution, the sodium ions are freely movable within the SAPparticles. In absorbent articles, such as diapers or training pants, theSAP particles typically absorb urine. Compared to distilled water, urinecomprises a relatively high amount of salt, which at least partly ispresent in dissociated form. The dissociated salts comprised by theurine make absorption of liquid into the SAP particles more difficult,as the liquid has to be absorbed against an osmotic pressure caused bythe ions of the dissociated salts. The freely movable sodium ions withinthe SAP particles strongly facilitate the absorption of liquid into theparticles, because they reduce the osmotic pressure. Therefore, a highdegree of neutralization can largely increase the capacity of the SAPparticles and the speed of liquid absorption.

Furthermore, a higher degree of neutralization typically reduces thematerials expenses and, consequently, also reduces the overallmanufacturing costs for SAP particles: Sodium hydroxide, which iscommonly used to neutralize the polymer, is typically less expansivecompared to acrylic acid. Hence, increasing the neutralization degreeincreases the amount of sodium hydroxide comprised by a given amount ofSAP. Consequently, less acrylic acid is required for making SAPs.Therefore, the method of one embodiment provides an economicalattractive way of making SAP particles.

In one embodiment, the reduction of undesired side-reactions during thesurface cross-linking process is advantageous. Surface cross-linkingknown in the prior art requires increased temperatures, commonly aroundor above 150° C. At these temperatures, not only surface cross-linkingis achieved, but also a number of other reactions take place, forexample, anhydride-formation within the polymer or dimer cleavage ofdimers previously formed by the acrylic acid monomers. Theseside-reactions are highly undesired, because they result in SAPparticles with decreases capacity.

As the surface cross-linking process according to one embodiment doesnot necessarily need increased temperatures but can also be carried outat moderate temperatures, those side-reactions are considerably reduced.According to one embodiment, the surface cross-linking reaction can beaccomplished at temperatures of less than about 100° C., in anotherembodiment at temperatures less than about 80° C., in another embodimentat temperatures less than about 50° C., in another embodiment attemperatures less than about 40° C., and yet in another embodiment attemperatures between about 20° C. and about 40° C. Drying of the SAP maybe carried out at temperatures above 100° C. but below 150° C.,preferably below 120° C., to avoid the undesired side reactions.

Also, at elevated temperatures around or above 150° C. commonly appliedin the surface cross-linking process known in the prior art, the SAPparticles sometimes change their color from white to yellowish. In oneembodiment, due to the reduced temperatures required for surfacecross-linking, the problem of color degradation of the SAP particles canbe considerably reduced.

The surface cross-linking according to one embodiment can be carried outtogether with one or more thermally activatable surface cross-linkersknown in the art, for example, 1,4-butandiol. In this embodiment,however, both, UV radiation and increased temperatures (typically above140° C.), are required. In these embodiments, the surface of theresulting SAP particles will further comprise the reaction product ofthe thermally activatable surface cross-linker.

In embodiments, wherein radical formers and/or surface cross-linkingmolecules are applied, the method of one embodiment may further comprisean optional washing step to wash off un-reacted surface cross-linkingmolecules and/or radical former molecules or to wash off moleculesformed by side reactions.

UV Irradiation

In one embodiment, the SAP particles are exposed to ultraviolet- (UV-)radiation. The UV-domain of the electromagnetic spectrum is definedbetween wavelengths of 100 and 380 nm and is divided into the followingranges: UV-A (315 nm-400 nm), UV-B (280 nm-315 nm), UV-C (200 nm-280 nm)and Vacuum UV (VUV) (100 nm-200 nm).

Use of VUV Radiation:

In one embodiment, xenon (Xe₂—) excimer radiation sources, pulsed orcontinuous, are applied. In contrast to well-known excimer lasers,excimer lamps emit quasi-monochromatic incoherent radiation. Generationof incoherent excimer radiation is made possible for example bymicrowave discharges or by dielectrically barrier discharges (DBD,silent discharges) in specific gas atmospheres.

In one embodiment, the Xe₂-emission shows a relatively broad band in theVUV spectral domain from 160 to 200 nm, peaking at a wavelength of 172nm with a full width at half maximum (FWHM, half-width) of 14 nm. In oneembodiment, the wavelength within the VUV spectrum is from 160 nm to 200nm, in another embodiment the wavelength has a peak at 172 nm.

A pulsed Xe₂-excimer radiation source suitable for laboratory studies isavailable under the trade name Xeradex™ (Osram, Munich, Germany) withelectrical powers of 20 W or 100 W. However, if the method of oneembodiment is used to surface cross-link SAP particles in amounts commonin industrial application, the power of the radiation source should beas high as 10 kW or even higher.

Continuous Xe₂-excimer radiation sources with electrical powers of up to10 kW can be purchased from Heraeus Noblelight, Hanau, Germany), smallersources are also available from Ushio Ltd. (e.g., Ushio Deutschland,Steinhöring).

Use of UV Radiation Having a Wavelength from 201 nm to 400 nm:

UV radiation within the UV-A, UV-B or UV-C range depending on thepresence, concentration and nature of a photo-initiator, commerciallyavailable mercury arcs or metal halide radiation sources can be used.The choice of the radiation source depends on the absorption spectrum ofthe radical initiator and possibly on geometry of the equipment used forsurface cross-linking. In one embodiment, the UV-B range proved to beeffective, in combination with the above-described initiators.

The radiation sources can be optionally cooled with gas, and, to thisend, may be embedded in or may contain a cooling sleeve.

The method of one embodiment may be carried out in a fluidized bedreactor having a radial symmetric geometry with a rod-shaped radiationsource in the centre or by using vibrating plates for UV exposure.

However, for the method of one embodiment, the Brønsted acids and—ifapplicable—the radical formers and surface cross-linking molecules haveto be homogeneously applied onto the SAP particles. Further, it has tobe ensured that all SAP particles are homogeneously exposed to the UVradiation, avoiding that individual SAP particles are shadowed for anoverly long period. Hence, the SAP particles have to be agitated whileexposed to UV radiation, which may be done, e.g., by rather gentle shearmovements or by more vigorous agitation.

In another embodiment, if VUV radiation is used, the method is carriedout under normal atmosphere to reduce costs. However, as under normalatmosphere VUV radiation is partly absorbed by oxygen, the range ofcoverage of the VUV radiation is restricted. Moreover, upon absorptionof VUV by oxygen, ozone is formed. Hence, it may be desirable to placethe process equipment into a ventilated container to avoid contact ofoperating personnel with ozone.

However, to increase the range of coverage of the VUV radiation (as theradiation is not absorbed by oxygen), the method of one embodiment canalso be carried out under nitrogen. The range of coverage of VUV innitrogen is much larger compared to the range of coverage of VUV innormal atmosphere. This allows for more leeway in equipment design andprocess layout.

Also, high degrees of atmospheric humidity should be avoided, as VUVradiation is also absorbed by water molecules, and the degree ofatmospheric humidity should be kept substantially constant over time toachieve a relatively constant degree of surface cross-linking. Tocontrol the degree of atmospheric humidity and to restrict atmospherichumidity to a relatively low level, the water content in the SAPparticles should be kept constant, preferably at a relatively low level.

In one embodiment, if UV radiation having a wavelength from 200 nm to400 nm is used and is carried out under normal atmosphere to reducecosts. Also, without wishing to be bound by theory, it is believed thatnormal atmosphere enables improved surface cross-linking results asoxygen, which is a bi-radical, may participate in the reaction mechanismby formation of intermediate peroxile radicals upon irradiation. Hence,the number of available radicals is proliferated, which in turn enablethe formation of carbon-centered radicals in the polymer backbone of thepolymer chains in the surface of the SAP particles. The degree ofhumidity is not crucial for UV irradiation having a wavelength from 200nm to 400 nm, as water molecules do not absorb within that range.

Brønsted Acids

For surface cross-linking with UV irradiation, using SAP particles witha relatively high degree of neutralization, Brønsted acids are able toconsiderably improve the surface cross-linking process.

In one embodiment, SAP particles with degrees of neutralization of 60%or above, namely from about 60% to about 95%, in another embodiment fromabout 65% to about 95%, in another embodiment from about 70% to about95%, and in yet another embodiment from about 75% to about 95% aresubjected to UV irradiation for surface cross-linking. To improve theeffectiveness of the method, Brønsted acids are applied onto the SAPparticles.

It is believed that the electron-drawing effect (known in the literatureas “-I effect”) of the carboxy groups (COOH) comprised by the polymer ofthe SAP particles contributes to the overall reaction speed andefficiency of the surface cross-linking method of one embodiment, thoughthey are not directly involved in the surface cross-linking reactions(see above). This is presumably due to an accelerated hydrogenabstraction from C—H groups positioned adjacent the carboxyl groups inthe backbone of the polymer, which results in the formation ofcarbon-centered radicals in the backbone of the polymer (see above).

However, for SAP particles having a relatively high degree ofneutralization, most of the carboxyl group are de-protonated (COO⁻), asthey are in the form of the corresponding carboxylate salt (COOM with Mbeing a monovalent metal cation such as Na⁺). The “-I effect” of thecarboxylate salt is known to be weaker compared to the protonated form.It has been found that this shortcoming of SAP particles with arelatively high degree of neutralization in light of surfacecross-linking with UV radiation can be compensated by adding a Brønstedacid without affecting the overall concept of neutralization. TheBrønsted acid is capable of releasing protons (H⁺), thereby transferringthe carboxylate salt in the surface of the SAP particle into theprotonated form COOH.

Also, if the carbon-centered radical in the backbone of the polymercomprised by the SAP particle is formed by decarboxylation, i.e., byabstracting a whole carboxyl group instead of abstracting a proton froma C—H group, the increased number of protonated COOH groups in thesurface of the SAP particles positively influences the effectiveness ofsurface cross-linking with UV radiation: COOH groups undergosignificantly more readily decarboxylation than COOM groups

By subjecting SAP particles with a relatively high degree ofneutralization to a treatment with one or more Brønsted acids, a lowdegree of neutralization can be selectively adjusted in the surface ofthe SAP particles, resulting in a more efficient reaction. At the sametime, these SAP particles still to have a relatively high degree ofneutralization in the core of the SAP particles, which is economicallyfavorable due to the advantages of a high neutralization degree asdescribed above.

Additionally to the Brønsted acid, a Lewis acid can be applied,preferably the aluminum cation Al³⁺, wherein Al³⁺ is preferably appliedin the form of aluminum sulfate Al₂(SO₄)₃.

A Brønsted acid is any organic or inorganic compound capable ofreleasing protons (H⁺). In one embodiment, Brønsted acids are mineralacids like hydrochloric acid, sulphuric acid, phosphoric acid; saturatedorganic carbonylic acid like acetic acid, lactic acid, citric acid,succinic acid; oligomeric or polymeric organic acids like low molecularweight poly acrylic acid having a molecular weight of from 50 to 5000g/mol and saturated inorganic acids. In one embodiment, a saturatedinorganic acid is boric acid. In another embodiment, Brønsted acids arethe mineral acids and the satured organic carbonylic acids. In yetanother embodiment, hydrochloric acid is the Brønsted acid.

The pK_(a) value (dissociation index) of the Brønsted acid should belower than the pK_(a) value of the conjugated acid of the SAP repeatunit, which—in case of poly(meth)acrylic acid as polymer in the SAPparticle—is typically between 4 and 5. In one embodiment, Brønsted acidshave a pK_(a) value of less than 5, in another embodiment less than 4,and in yet another embodiment less than 3. For example, HCl has a pK_(a)value of −6.

However, apart from the pKa value, the effect of the acid on theparticle flow behavior of the SAP particles during the irradiation mayalso influence the Brønsted acid, which is finally selected for themethod of one embodiment. Some Brønsted acids may result onagglomeration of the SAP particles while other may even have a positiveeffect on the fluidity properties of the SAP particles (and may thus actas fluidity enhancers). The selection of the appropriate Brønsted acidtherefore may have to be made depending on the given circumstances.

The amount of the Brønsted acid applied in one embodiment is in therange of from about 0.005 weight-% to about 10 weight-% by weight of SAPparticles, in another embodiment from about 0.01 weight-% to about 5.0weight-%, and in yet another embodiment from about 0.1 weight-% to about3.0 weight-%. The amount of Brønsted acid also depends on the Brønstedacid which is used, on the radical former (if applied) and on thesurface cross-linking molecules (if applied). Generally, if a relativelyweak Brønsted acid or a Brønsted acid with a relatively high molecularweight is applied, the amount of Brønsted acid should be higher comparedto a stronger Brønsted acid or to a Brønsted acid having a lowermolecular weight. Also, Brønsted acids having a higher pK_(a) value willbe required in lower amounts compared to a Brønsted acid with a higherpK_(a) value. HCl, in one embodiment is applied in the range of fromabout 0.1 weight-% to about 1.0 weight-% by weight of the SAP particles.

In principle, also a mixture of several Brønsted acids can be used.However, this is less preferred as it increases the overall complexityof the method.

In one embodiment, the Brønsted acid is applied in water as an aqueoussolution, as an emulsion or a suspension, which may also comprise the UVactivatable radical former and the surface cross-linking molecules (ifapplied). A typical concentration of the Brønsted acid in an aqueoussolution is 1 mol/l to 2 mol/l. Alternatively, the Brønsted acid canalso be applied separately from the radical former and/or surfacecross-linking molecules (if applied).

Also, the Brønsted acids can be applied while dissolved or suspended inalcohol, for example, isopropanol. The advantage of using alcoholinstead of water is that alcohol does not migrate into the SAP particlesto a substantial degree. Hence, it is easier to control the penetrationdepth in order to avoid Brønsted acids migrating onto the core. Therebyit is easier to ensure that the surface cross-linking reaction isactually restricted to the surface of the SAP particles. The alcohol maybe removed (via evaporation) prior to UV irradiation of the SAPparticles.

If the Brønsted acids are applied in a mixture of alcohol and water, thepenetration depth of the mixture—and thereby of the Brønsted acids—canbe carefully adjusted by choosing the appropriate ratio between alcoholand water.

It may also be desirable to apply the Brønsted acid suspended in water,choosing a Brønsted acid which does not dissolve in water very well.Thereby it is also possible to ensure that the Brønsted acids actuallyremain in the surface of the SAP particles and do not migrate into thecore together with the water.

The Brønsted acid can be applied onto the SAP particles prior to UVirradiation. In one embodiment, if the Brønsted acid is applied inwater, it is applied immediately before UV irradiation takes place toensure that the Brønsted acid does not migrate into the core to asubstantial degree. In one embodiment, the Brønsted acid should not beapplied more than 10 minutes prior to UV irradiation, in anotherembodiment, not more than 5 minutes, and in yet another embodiment, thetime between application of the Brønsted acid and UV irradiation shouldnot be more than 1 minute, especially if the Brønsted acid is applied inwater.

As the SAP particles have a buffering effect for the Brønsted acid, itmay be desirable to apply the Brønsted acid continuously during UVirradiation, for example, via spraying to ensure a permanent surplus ofBrønsted acid. The Brønsted acid can be applied continuously during UVirradiation in addition to an initial application of Brønsted acid priorto UV irradiation or can be applied only during UV irradiation.

Fluidity enhancers, as they are widely known in the art, such ashydrophilic amorphous silicas, as they are commercially available, forexample, from Degussa Corp., can optionally be added to the SAPparticles to assist in avoiding agglomerates, for example, if the watercontent of the SAP particles is relatively high. The fluidity enhancersare typically applied in a range of from about 0.1 weight-% by weight ofSAP particles to about 10 weight-% by weight of SAP particles.

Absorbent Articles

In one embodiment, the SAP particles made by the method are applied inabsorbent cores of absorbent articles. As used herein, absorbent articlerefers to devices that absorb and contain liquid, and more specifically,refers to devices that are placed against or in proximity to the body ofthe wearer to absorb and contain the various exudates discharged fromthe body. Absorbent articles include but are not limited to diapers,adult incontinent briefs, diaper holders and liners, sanitary napkinsand the like.

In one embodiment, absorbent articles are diapers. As used herein,“diaper” refers to an absorbent article generally worn by infants andincontinent persons about the lower torso.

In one embodiment, absorbent articles typically comprise an outercovering including a liquid pervious topsheet, a liquid imperviousbacksheet and an absorbent core generally disposed between the topsheetand the backsheet. The absorbent core may comprise any absorbentmaterial that is generally compressible, conformable, non-irritating tothe wearer's skin, and capable of absorbing and retaining liquids suchas urine and other certain body exudates. In addition to the SAPparticles, the absorbent core may comprise a wide variety ofliquid-absorbent materials commonly used in disposable diapers and otherabsorbent articles such as comminuted wood pulp, which is generallyreferred to as air felt.

Exemplary absorbent structures for use as the absorbent assemblies aredescribed in U.S. Pat. No. 5,137,537 entitled “Absorbent StructureContaining Individualized, Polycarboxylic Acid Crosslinked Wood PulpCellulose Fibers” which issued to Herron et al. on Aug. 11, 1992; U.S.Pat. No. 5,147,345 entitled “High Efficiency Absorbent Articles ForIncontinence Management” issued to Young et al. on Sep. 15, 1992; U.S.Pat. No. 5,342,338 entitled “Disposable Absorbent Article ForLow-Viscosity Fecal Material” issued to Roe on Aug. 30, 1994; U.S. Pat.No. 5,260,345 entitled “Absorbent Foam Materials For Aqueous Body Fluidsand Absorbent Articles Containing Such Materials” issued to DesMarais etal. on Nov. 9, 1993; U.S. Pat. No. 5,387,207 entitled “Thin-Until-WetAbsorbent Foam Materials For Aqueous Body Fluids And Process For MakingSame” issued to Dyer et al. on Feb. 7, 1995; U.S. Pat. No. 5,397,316entitled “Slitted Absorbent Members For Aqueous Body Fluids Formed OfExpandable Absorbent Materials” issued to LaVon et al. on Mar. 14, 1995;and U.S. Pat. No. 5,650,222 entitled “Absorbent Foam Materials ForAqueous Fluids Made From High Internal Phase Emulsions Having Very HighWater-To-Oil Ratios” issued to DesMarais et al. on Jul. 22, 1997.

Test Methods

The capacity of the SAP particles is often described in terms of thecentrifuge retention capacity value (CRC). A test method for CRC isdescribed in EDANA method 441.2-02.

The parameter commonly used to describe the behavior of SAP particlesunder a certain pressure is AAP (absorbency against pressure). AAP ismeasured according to EDANA method 442.2-02, using a pressure of 4.83kPa.

Permeability of the gel bed comprised of SAP particles is generallymeasured as saline flow conductivity (SFC). A test method to determineSFC is described in U.S. Pat. No. 5,562,646, issued to Goldman et al. onOct. 8, 1996. In one embodiment, the test method in U.S. Pat. No.5,562,646 is modified in that a 0.9% NaCl solution is used instead ofJayco solution).

EXAMPLES

Base Polymer:

As base polymer, the water-swellable polymer as described in Example 1.2of WO 2005/014066 A1, titled “Absorbent articles comprising coatedwater-swellable material” and filed on 17 Feb. 2005 is used. However,the neutralization degree of the base polymer, which is 75% in Example1.2 of WO 2005/014066 A1 has been adjusted to 70% and 85%, respectively,as required by the Examples herein. Also, the amount of MBAA has to beroutinely adjusted accordingly to obtain SAP particles with a CRC valueof 30.5 g/g (Example 1) and 31 g/g (Example 2). It should be noted, thatthe CRC value can principally be adjusted in the same way as the CCRCway, which is described in Example 1.2 of WO 2005/014066 A1.

Example 1

3 parts of the radical former sodium peroxodisulfate, and 0.6 parts ofthe Brønsted acid HCl are dissolved in 7 parts of water.

100 parts (per weight) of non-surface cross-linked SAP particlesconsisting only of the base polymer described above and having a degreeof neutralization of 70% are mixed with the aqueous solution comprisingthe radical former sodium peroxodisulfate and the Brønsted acid HClunder vigorous stirring. Mixing is done for 10 minutes.

Immediately thereafter, the SAP particles are subjected to UVirradiation for 10 minutes, using a 2 kW Medium Pressure Mercury Lamp.The distance between the irradiation source and the SAP particles is assmall as possible and in the current example is about 10 cm. The mixingcontinued throughout the irradiation step. Mixing and irradiation arecarried out under normal atmosphere.

The 100 parts of SAP particles correspond to 10 g and the SAP particleshave a particle size distribution of from 150 μm to 850 μm.

Comparative Example 1

Comparative example 1 corresponds to Example 1 with the only differencethat no Brønsted acid has been added.

Example 2

Example 2 differs from Example 1 in that the SAP particles consistingonly of the base polymer described above have a degree of neutralizationof 80%. Also, 8 parts of water have been added instead of 7 parts.Otherwise, Example 2 does not differ from Example 1.

Comparative Example 2

Comparative example 2 corresponds to Example 2 with the only differencethat no Brønsted acid has been added.

The SFC, AAP and CRC values for these Examples are summarized in Table1:

Example 3

Example 3 differs from Example 1 in that instead of applying HCl asBrønsted acid, 1.6 parts of the Brønsted acid H₂SO₄ are applied. Also, 8parts of water have been added instead of 7 parts. Otherwise, Example 3does not differ from Example 1.

The SFC, AAP and CRC value of the initial SAP particles consisting onlyof the base polymer (70% neutralized and 85% neutralized, respectively)and of the SAP particles of Examples 1, 2 and 3 and Comparative Examples1 and 2 after they have been subjected to the test is determinedaccording to the test methods set out above. TABLE 1 UV surface AAPcross- CRC at 4.83 kPa SFC linking Brønsted acid Neutralization (g/g)(g/g) (10⁻⁷ cm³ s g⁻¹) Base polymer No none 70% 30.5 6.9 0 Example 1 YesHCl 70% 24.8 19.5 64 Comparative Yes none 70% 26.2 18.9 24 Example 1Example 3 Yes H₂SO₄ 70% 25.2 18.8 48 Base polymer No none 85% 31.0 6.0 0Example 2 Yes HCl 85% 26.3 17.1 11 Comparative yes None 85% 27.1 13.2 3Example 2

For SAP particles without surface cross-linking (hence, only consistingof the base polymer), the CRC value is typically rather high as the SAPparticles are not restricted in swelling due to the cross-linksintroduced on the surface of the SAP particles. After surfacecross-linking, the CRC value of the SAP particles decreases.

In contrast, the AAP and SFC values for non surface cross-linked SAPparticles is very low (for the SFC, the value can be as low as zero): Asthe SAP particles are extremely soft, they easily deform under pressure(=low AAP value). Consequently, gel blocking occurs, which results in avery low SFC value.

Generally, an increase in AAP and SFC value and a decrease in CRC valuecompared to the non surface cross-linked SAP particles consisting onlyof the base polymer is an indirect proof that surface cross-linking hasactually taken place.

As a result, the Examples show that the base polymer has indeed beensurface cross-linked by the method described herein.

The dimensions and values disclosed herein are not to be understood asbeing strictly limited to the exact numerical values recited. Instead,unless otherwise specified, each such dimension is intended to mean boththe recited value and a functionally equivalent range surrounding thatvalue. For example, a dimension disclosed as “40 mm” is intended to mean“about 40 mm”.

All documents cited in the Detailed Description of the Invention, are,in relevant part, incorporated herein by reference; the citation of anydocument is not to be construed as an admission that it is prior artwith respect to the present invention.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

1. An absorbent article comprising surface cross-linked superabsorbentpolymer particles, said superabsorbent polymer particles being made by amethod which comprises the steps of: a) providing superabsorbent polymerparticles having a surface and a core and having a degree ofneutralization of more than 60 mol-%; b) applying one or more Brønstedacids onto the surface of the superabsorbent polymer particles; andeither c1) exposing said superabsorbent polymer particles to irradiationwith vacuum UV radiation having a wavelength from about 100 nm to about200 nm or c2) exposing said superabsorbent polymer particles toirradiation with UV radiation having a wavelength from about 201 nm toabout 400 nm and wherein further to the Brønsted acids, radical formermolecules are applied to the surface of the superabsorbent polymerparticles.
 2. The absorbent article of claim 1, wherein in step c1 ofsaid method further to the Brønsted acids, radical former molecules areapplied to the surface of the superabsorbent polymer particles.
 3. Theabsorbent article of claim 1, wherein in said method additionallysurface cross-linking molecules are applied to the surface of thesuperabsorbent polymer particles and wherein the surface cross-linkingmolecules comprise at least two functional groups, said functionalgroups being C═C double bonds or being CH—X moieties, with X being ahetero atom.
 4. The absorbent article of claim 1, wherein in said methodthe Brønsted acid is selected from the group consisting of hydrochloricacid, sulphuric acid and phosphoric acid.
 5. The absorbent article ofclaim 1, wherein in said method the Brønsted acid is applied in aconcentration of from about 0.005 weight-% to about 10 weight-% byweight of the SAP particles.
 6. The absorbent article of claim 1,wherein in said method the Brønsted acid is applied continuously duringUV irradiation of the superabsorbent polymer particles.
 7. The absorbentarticle of claim 1, wherein said method is carried out at a temperatureof less than about 100° C.
 8. The absorbent article according to claim1, wherein the absorbency against pressure (AAP) of about 4.83 kPa ofthe superabsorbent polymer particles increases by at least about 1 g/gafter the superabsorbent polymer particles have been subjected to saidmethod.
 9. The absorbent article according to claim 1, wherein thesaline flow conductivity (SFC) of the superabsorbent polymer particlesincreases by at least about 10 10⁻⁷ cm³ s g⁻¹ after the superabsorbentpolymer particles have been subjected to said method.